And how will the government deal with a fine non-payer who keeps their wealth in physical form? Say they have no money in the bank but keep gold bars in a stack under their bed. They barter for everything they need.

At some point it devolves to burly men with or without guns making people do things. Or women in this enlightened age. In practice it hopefully isn't a common thing but there has to be a level of physical compulsion backing up everything that communities don't enforce among each other anyway. A major job of people in Congress is to work out exactly what does happen when people refuse to comply and at some point it gets physical (quite quickly if someone is really bent on being uncooperative). Issues like that would be close to the top of every politicians's mind.